1. Field of the Invention
The present invention relates to an oxygen concentration sensor, more particularly to an oxygen concentration sensor with a modified alumina coating layer on the surface of the sensor.
2. Description of the Prior Art
Oxygen concentration sensors are used in various fields, particularly as a sensor for controlling exhaust gas from an engine in an automobile. The oxygen concentration sensors include an oxygen concentration differential cell type, utilizing zirconia (ZrO.sub.2) solid electrolyte, and a resistance variation-detecting type, utilizing titania (TiO.sub.2) semiconductor.
Such oxygen concentration sensors typically have a porous alumina coating layer on which a catalyst of Pt/Rh is carried. The porous alumina coating layer traps oil components and so forth and the catalyst of Pt/Rh accelerates equilibration of unburnt components such as hydrocarbons and carbon monoxide in the exhaust gas. The porous alumina coating layer is formed by sintering at about 1100.degree. C. The catalyst is then carried on the porous alumina coating layer by dropwise applying a solution containing Pt chloride and Rh chloride to the coating layer or dipping the coating layer in the solution and heating them to reduce the Pt and Rh chlorides, by which fine Pt and Rh particles are deposited on the porous alumina coating layer.
The oxygen concentration sensors having a catalyst-carrying alumina coating layer have an excellent performance at the beginning, but the output of the sensors and the reaction speed are lowered after use for a long time.
The object of the present invention is to provide an oxygen concentration sensor which can have a normal output for a long time, or a prolonged life time.